Phosphate rock is any ore that contains one or more phosphatic minerals of sufficient purity and quantity to permit its commercial use as a source of phosphatic compounds or elemental phosphorous. This definition is essentially economic rather than geologic, for if a rock can be utilized for the indicated purpose it is a phosphate rock. Phosphate rock includes two major subdivisions, the first consisting of crystalline rock with enough of the mineral apatite to be of commercial interest in providing a source of calcium and phosphorous. Fluorapatite is calcium fluorophosphate of the formula Ca.sub.4 (PO.sub.4).sub.6.CaF.sub.2, and is widely distributed in many kinds of rock. The second consists of sedimentary phosphate rock known as phosphorite, the essential mineral of phosphorite being carbonate apatite. In the art, phosphate rock is essentially synonymous with phosphorite.
Phosphate rock of any type is utilized for its phosphatic content with its physical properties being of no concern except in mining and processing. The phosphate content of the rock or ore is expressed in either of two ways. One is the percentage of Bone Phosphate of Lime or "BPL", which is tricalcium phosphate, Ca.sub.3 (PO.sub.4).sub.2. The BPL content of beneficiated phosphate rock will generally range from about 50 to 85 percent. The second manner of expressing phosphate content is in terms of phosphorous pentoxide, P.sub.2 O.sub.5. The ratio of BPL content to P.sub.2 O.sub.5 content is about 2.18 to 1. Thus, a rock with 60% BPL content contains about 27.5% P.sub.2 O.sub.5.
As a practical matter, most modern commercial beneficiation plants are designed to process and upgrade phosphate ore to remove slimes and sands so as to provide a phosphate rock in which the BPL content has been upgraded.
Phosphate rock is obtained from underground mining or surface mining of phosphate ore, the latter initially involving the use of large drag line excavators which remove the overburden and then recover the crude phosphate ore which is known as "matrix". The matrix is then washed by hydraulic jets in an improvised sump pit and the resulting mixture of phosphate ore, water, sand and gravel, called the "slurry", is then pumped by pipeline to a washing plant. In the washing plant the phosphate ore is sized and concentrated so that an original matrix which could have a BPL content in the range of 20-30% may be upgraded to a phosphate rock having a higher BPL content, e.g. in the range of 70% or higher. In the treatment plant, the matrix is initially washed to provide a pebble product and a flotation feed, with slimes having been removed. The slimes are discarded and the pebble may be suitable for sale or may be blended with other high grade concentrate. The flotation feed is passed to a flotation plant where a high-grade concentrate product is separated from sand tailings. The slimes and sand are normally discarded into slime ponds and the like.
Of the products from the beneficiation plant, only the concentrate will normally have a sufficiently high BPL content to be used in commercial phosphoric acid plants for the production of phosphoric acid and/or the production of other useful materials containing calcium and phosphorous. Accordingly, most phosphoric acid plants are suitable only for handling a feedstock having a relatively high BPL content such as the concentrate product which is obtained from flotation.
The pebble product has a size ranging from 1/4" to +14 mesh and a BPL value ranging from 38 to about 68 (17.4-31.2% P.sub.2 O.sub.5). With the declining quality of rock, in recent years small amounts of pebble have been ground and blended into higher quality, beneficiated rock. In the past, pebble having a BPL value higher than 58 (26.6% P.sub.2 O.sub.5) was stored while the lower grade, high alumina pebble was discarded with the tailings.
The pebble is obtained by screening "as mined" rock and retaining the +14 mesh or "pebble" fraction, but in recent years some pebble has been retained from screening at +10 mesh.
In spite of the tremendous reserve of P.sub.2 O.sub.5 which the high alumina pebble represents, it is not processed because of its high content of metal compounds such as compounds of calcium, magnesium, aluminum and iron. This is due in part to a need to produce phosphoric acid of low metallic ion content that can be readily converted to superphosphoric acid (70-76% P.sub.2 O.sub.5) for use in liquid fertilizer manufacture.
When high alumina phosphate pebble rock is digested with phosphoric acid and sulfuric acid to form aqueous phosphoric acid and calcium sulfate by the usual gypsum processes, a great many soluble metallic phosphates are formed. When the phosphoric acid is concentrated to merchant grade phosphoric acid and superphosphoric acid, after removal of the solid calcium sulfate and gangue and after clarification, the presence of the soluble metallic phosphate values greatly increases the viscosity of the phosphoric acid. The viscosity of the phosphoric acid can increase to such an extent that the phosphoric acid cannot be handled or transported as a fluid. As the concentration of phosphoric acid is increased, the solubility of the metallic phosphates sharply decreases to form sludges, scale and complex precipitates in phosphoric acid. These sludges settle out in the phosphoric acid creating storage, handling and transportation problems for the acid. In addition, formation of sludges, scale and precipitates causes an appreciable loss of the P.sub.2 O.sub.5 values in the phosphoric acid.
In Central Florida, the pebble is coarser, generally has a BPL value around 68 (31% P.sub.2 O.sub.5) and is of lower metallic content than Northern Florida pebble. This pebble presently has a market value because it may be used as a feed to electric furnaces to produce elemental phosphorous and can also be used with phosphate rock within restrictions imposed by product quality standards.
However, high alumina pebble inventory in Central Florida also increased with increased emphasis on low metallic content phosphoric acid which can be converted to high quality superphosphoric acid. This made even more remote the processing of high alumina pebble by the usual gypsum type wet process, in both Central and certainly Northern Florida.
Beneficiated phosphate rock also contains metallic impurities, though less than a high alumina pebble, which are undesirable and complicate the production of superphosphoric acid due to the metallic phosphate complexes that are formed which increase the viscosity of the acid and form sludges which settle and can result in a P.sub.2 0.sub.5 loss if not recycled.
There has been substantial work in the art in an effort to upgrade phosphate ore or matrix and/or utilize low-grade phosphatic materials in order to obviate the requirements of the expensive beneficiation processes and loss of P.sub.2 O.sub.5 content. For example, U.S. Pat. Nos. 2,143,865, 3,391,993, 4,042,666, and 4,113,184 describe various methods for treatment of phosphate rock prior to acidulation with sulfuric acid in a conventional phosphoric acid plant. In all of these prior patents, the phosphate ore is treated as with steam as in U.S. Pat. No. 2,143,865 to eliminate fluorine; with water and a defoaming agent as in U.S. Pat. No. 3,391,993 in producing dicalcium phosphate; formed into an aqueous slurry with a viscosity reducing agent as in U.S. Pat. No. 4,042,666 to reduce clay-swelling problems; or, combined with an additive for wet grinding of the rock as in U.S. Pat. No. 4,113,184.
In U.S. Pat. No. 2,571,866, there is described a process for the production of phosphate concentrate from Florida phosphate rock in which water is used to separate the slime or clay from the larger useful particles of phosphate followed by further processing to produce a phosphate concentrate. In U.S. Pat. No. 4,105,749, phosphoric acid is produced from a phosphate ore matrix which has been slurried with an organic solvent to remove impurities prior to acidulation with sulfuric acid.
British Pat. No. 852,538 discloses a process for enriching natural phosphates by reacting phosphate ore at temperatures of no higher than 50.degree. C. with a dilute phosphoric acid solution which will impregnate the ore and cause impurities to flow away in the form of a solution. This treatment is to enrich the proportion of lime in the original ore and remove slimes and the like.
Phosphate rock feed of sufficiently high BPL content is currently processed in so-called wet process phosphoric acid plants utilizing a basic and well-known procedure for the acidulation of the phosphate rock by reaction of the rock with sulfuric acid to form phosphoric acid with subsequent reaction of the phosphoric acid, with for example, ammonia to produce monoammonium phosphate (MAP) and diammonium phosphate (DAP). The phorphoric acid formed in this process is marketable wet process phosphoric acid. In this reaction, a by-product is gypsum having the chemical formula CaSO.sub.4.2H.sub.2 O. The BPL content of the rock used as feedstock in such processes will range from about 58% to about 68% BPL.
In these systems, the conventional wet process phosphoric acid technology accomplishes two primary objectives, namely: (1) phosphate rock acidulation, and (2) the growth of readily filterable calcium sulfate crystals either as the dihydrate (gypsum), or as the hemihydrate. Conventional phosphoric acid technology carries out both of these objectives essentially simultaneously which leads to a number of environmental and purification problems. The presence of excess strong sulfuric acid in the acidulation phase releases fluorides as HF, SiF.sub.4 and/or H.sub.2 SiF.sub.6. This poses serious fluoride emission and subsequent recovery problems. Furthermore, unless excess sulfate levels are carefully and closely controlled, minute gypsum crystals can and will blind rock particles and usually result in poor P.sub.2 O.sub.5 recovery. The presence of free H.sub.2 SiF.sub.6 in the acid system leads to severe scaling and excessive maintenance costs even with improved design features to minimize this effect.
Prior art is also known which acidulates phosphate rock with phosphoric acid and then recovers solid monocalcium phosphate by cooling the resulting solution and recovering the monocalcium phosphate. Processes of this type are disclosed, for example, in U.S. Pat. Nos. 2,567,227, 2,728,635, 3,494,735, and 3,645,676. None of these prior art processes, however, are concerned with low grade feedstocks.
In the above-identified previously issued U.S. Pat. Nos. 4,086,322 and 5,160,657 of this assignee, there are disclosed processes by which phosphate rock may be acidulated with phosphoric acid in the presence of potassium ion and silicon dioxide. These processes are useful as effective procedures for the elimination of fluoride evolution. The latest technology concerning this problem is the above-mentioned U.S. Pat. No. 4,160,657, which represents a departure from prior processes in providing for more economic utilization of potassium fluosilicate in the system wherein both phosphoric acid and potassium ion are regenerated and reused as essential reactants.
U.S. Pat. Nos. 3,619,136 and 3,792,151 to Case disclose the reaction of phosphate rock with recycle phosphoric acid at temperatures of about 125.degree.-180.degree. F. (52.degree.-83.degree. C.) to form a solution of monocalcium phosphate in phosphoric acid, removing insolubles, and reacting the phosphoric acid solution with sulfuric acid to produce phosphoric acid and hydrated calcium sulfate, separating the hydrated crystals and recycling at least a portion of the phosphoric acid to the phosphate rock acidulation. These patents point out that under the conditions cited, fluorides are not substantially evolved but remain primarily unreacted and may be found with insoluble materials although a portion remains in the phosphoric acid solution product. These patents state that any grade of phosphate rock can be utilized in the process including mine run rock and Florida pebble rock. This patentee attempts to remove the slimes contained in low grade rock by settling and filtration. The working examples show treatment only of 70 BPL rock.
A substantial problem faced by the prior art in processing low-grade phosphate rock for the production of phosphoric acid and other products is the relative substantial proportion of slimes, sand and other components contained in the rock. These materials interfere in processing of the rock and particularly in the ability to obtain filterable gypsum crystals from the reaction product so as to provide the phosphoric acid material. Substantial prior teachings are available which show that the art has attempted to overcome this problem by various procedures to separate the silica, slimes and other insolubles. For example, U.S. Pat. Nos. 2,899,292 and 2,914,380 to Vickery describe processes wherein the phosphate rock is crushed, dissolved in phosphoric acid, and then attempts made to remove the silica and other insolubles in a separator. In U.S. Pat. No. 2,954,275 to Carothers et al., lime, which is more alkaline than phosphate rock, is added to phosphoric acid solutions and the mixture is then cooled so that a mixture of monocalcium phosphate and impurities are separated.
In U.S. Pat. No. 3,150,957 of Seymour et al, and in unpublished work by Seymour et al, low grade phosphate rock is acidulated with phosphoric acid and the impurities are decanted from the reaction mixture to provide a high grade phosphate rock. Thus, this process effects an initial partial digestion of the rock to extract monocalcium phosphate and other soluble and colloidal phosphates from the rock in fluid form. The unreacted rock residue may then be treated conventionally.
U.S. Pat. No. 4,284,614 to Ore describes a process wherein high alumina phosphate rock such as pebble, with or without comminution, is digested in phosphoric and sulfuric acid and the resultant phosphoric acid contains the metallic ions normally present in the treated rock and pebble. The metallic ions are then extracted from the acid by ion exchange with a water-immiscible organic sulphonic acid compound. After phase separation the organic phase containing the extracted metallic ions can be regenerated.
U.S. Pat. No. 3,919,395 describes an extraction process for the recovery of phosphorus compounds from both high and low grade phosphate ores, especially apatite-containing ores, using room temperature extraction of coarsely ground ore with dilute mineral acids in order to remove dissolved R.sub.2 O.sub.3 impurities from the ore to upgrade the ore. In this patent, the low grade rock is initially treated with a dilute mineral acid, separating the spent acid which contains colloidol silicates, fines, and dissolved phosphates, and then reacting the residue with a stronger acid. In related U.S. Pat. No. 4,029,743, phosphate rock is acidulated with a mixture of phosphoric acid, sulfuric acid and water in a first mix tank and with sulfuric acid in a second mix tank, in the production and recovery of phosphoric acid. Gypsum is also recycled to the first attack stage. The patentee suggests that low grade rock can be used in the process.
Other patents are known which treat phosphate rock in order to remove slimes, silicas and other insolubles by various methods including the use of water and weak phosphoric acid. However, none of these patents describe a method whereby the rock can be placed in a form suitable for subsequent acidulation by phosphoric acid to recover valuable products wherein the insolubles are removed from the system and fluoride evolution is substantially reduced.
In none of this prior work are there described processes by which very low grade phosphate rock or ore such as matrix is processed in a wet process phosphoric acid plant to produce marketable wet process phosphoric acid, together with other useful and marketable products. The present invention meets this need in the art.